The present invention relates to optical filtering techniques and in particular, in the preferred embodiment discloses utilising the filtering to produce a narrow transmission band pass filter.
Fibre Bragg gratings are proving to be instrumental in enabling the introduction of WDM systems. They offer highly selective band reject filtering needed for the transmission of multiple closely spaced wavelengths. These gratings, however, tend to work in reflection modes only since the large k-vector allows coupling from forward travelling modes to backward travelling modes. This can be a major disadvantage since to utilise the reflection mode in transmission systems requires the use of optical recirculators.
Known gratings which operate using phase-matching in the forward direction tend to have long periods, determined by the beat length between two modes. Fundamental mode conversion normally requires stripping of the higher order mode to achieve a loss bandpass. This is more readily achieved when conversion is to cladding modes. Whilst these filters have very low reflections, they operate as loss filters and not transmission bandpass devices. Further, the loss bandwidth tends to be broad, on the order of 20 nm, because of the small differences in modal dispersion. The spectral response of these filters is a rejection notch in transmission which is much broader than that of a Bragg grating. On the other hand, grating dispersion has been used to achieve mode conversion in reflection over a small wavelength range. In all these cases transmission notch filters are generally produced, whereas in most cases the oppositexe2x80x94a transmission bandpass filterxe2x80x94is generally desired.
It is an object of at least preferred embodiments of the present invention to utilise a Bragg grating to achieve effective mode coupling in a manner which allows for a narrow transmission bandpass filter to be produced.
In accordance with a first aspect of the present invention there is provided a method of creating an optical filter for selectively filtering an optical signal transmitted through said filter, said method comprising constructing a Bragg grating in an optical waveguide, the waveguide being coupled to the optical signal for guiding the optical signal through the grating; and adjusting said coupling so as to produce a selective transmission spectrum of the optical filter in accordance with predetermined requirements.
In accordance with a second aspect of the present invention there is provided a method of creating an optical filter for selectively filtering an optical signal transmitted through said filter, said method comprising constructing a Bragg grating in an optical waveguide, the waveguide being coupled to the optical signal for guiding the optical signal through the grating, and optimising said coupling so as to transfer light from an input mode to a predetermined output mode in accordance with requirements.
Preferably said filter comprises a narrow transmission band pass filter and the optical filter can be constructed as a planar waveguide. One form of construction of the grating can be tapered.
In accordance with a third aspect of the present invention there is provided an optical transmission filter comprising:
a tapered Bragg grating,
input coupling means for coupling light into one end of said grating;
output coupling means for coupling light out of said grating;
wherein the position of said input and output coupling means are adjust so as to produce one or more filter bandpass transmission peaks in accordance with predetermined requirements.
The filter can be utilized in cascade with other filter devices. The filter can further comprise a second output coupling means to couple light not coupled by the first output coupling means. The filter can be coupled with other optical elements including at least one of a phase shifting structure or a long period grating. Alternatively, the filter can be coupled with a non-linear optical element. The filter can also be utilized in a distributed feedback laser.